When E. coli is subjected to an energy source shift-down, protein synthesis is inhibited and a time-dependent disaggregation of polyribosomes and concomitant accumulation of complexed "monosomes" is observed. This redistribution is correlated with a specific decrease in the rate of peptide chain initiation and an increased functional stability of mRNA. Neither the rate of peptide chain propagation nor the rate of protein turnover increases. Electron microscopy has shown that the monosomes consist of single (70S) ribosomes lying at or near the 5'-ends of mRNA strands. The length distribution of the strands indicates that they are a reasonable representation of the total E. coli mRNA population. Preliminary results indicate that the complexes also contain about one mole of tRNA and we are engaged in determining the amino acid acceptor activity of this tRNA. Comparisons of protein synthesis (both initiation and propagation), mRNA lifetimes, ribosome distributions and protein turnover are being made in down-shifted cells and carbon-starved cells, as well as for cells in balanced growth at various rates in the chemostat. By these studies we hope to determine the role of energy metabolism in these processes and the mechanisms by which macromolecular metabolism is coordinated with the overall rate of cell growth. Further studies will be undertaken to determine whether the monosome complexes are capable of supporting in vitro protein synthesis and whether the proteins synthesized by these complexes represent a special subset of E. coli proteins.